Appropriate conditions to record activation of fast Ca2+ channels in frog skeletal muscle (Rana pipiens)

Effects of isoflurane on voltage-dependent calcium fluxes in rabbit T-tubule membranes: comparison with alcohols

The effects of racemic (+/-) and (+)- and (-)-stereoisomers of isoflurane on depolarization-induced (45)Ca(2+) fluxes mediated by voltage-dependent Ca(2+) channels were investigated in transverse tubule membrane vesicles from rabbit skeletal muscle. In the concentration range 0.5 to 2 mM, (+/-)-isoflurane inhibited (45)Ca(2+) fluxes and functionally modulated the effects of the Ca(2+) channel antagonist nifedipine (1-10 microM). Isoflurane-induced inhibition of (45)Ca(2+) fluxes was not significantly affected by pretreatment with either pertussis toxin (5 microg/ml) or phorbol 12-myristate 13-acetate (50 nM). Further experiments indicated that there were no significant differences between (+)- and (-)-stereoisomers of isoflurane with respect to the extent of inhibition of (45)Ca(2+) fluxes. Radioligand binding studies indicated that racemic and (+)- and (-)-isoflurane were equally effective in displacing the specific binding of [(3)H]PN 200-110 to transverse tubule membranes. There were no apparent differences between the effects of (+)- and (-)-isoflurane on the characteristics of [(3)H]PN 200-110 binding. Although the concentrations of isoflurane for the inhibitions of (45)Ca(2+) fluxes and radioligand bindings were similar, the concentrations of n-alcohols required for the inhibition of (45)Ca(2+) fluxes were lower than those for the displacement of radioligand. Comparison of the data for the displacement of [(3)H]PN 200-110 binding and the inhibition of (45)Ca(2+) fluxes by isoflurane and by n-alcohols suggested that both isoflurane and n-alcohols may have more than a single binding site. In conclusion, results indicate that isoflurane, independent of intracellular Ca(2+) levels, nonstereospecifically inhibits the function of voltage-dependent Ca(2+) channels and this effect is mediated through multiple binding sites.

Direct inhibition of voltage-dependent Ca(2+) fluxes by ethanol and higher alcohols in rabbit T-tubule membranes

The effects of ethanol and higher alcohols on 45Ca(2+) fluxes, mediated by voltage-dependent Ca(2+) channels (VDCCs), were investigated in inside-out transverse (T)-tubule membrane vesicles from rabbit skeletal muscle. 45Ca(2+) effluxes were induced by membrane potentials generated via establishing K(+) gradients across the vesicles, and were significantly inhibited by the inorganic Ca(2+) channel blocker La(3+) (1 mM) and the Ca(2+) channel antagonist nifedipine (1-10 microM). Ethanol, in the concentration range of 100-400 mM, caused a significant suppression of depolarization-induced 45Ca(2+) fluxes. Ethanol also functionally modulated the effect of nifedipine (1-10 microM) and the Ca(2+) channel agonist Bay K 8644 (1 microM) on Ca(2+) effluxes. Pretreatment with pertussis toxin (5 microg/ml) or phorbol 12-myrstate 13-acetate (PMA, 50 nM) did not affect the ethanol inhibition of 45Ca(2+) fluxes. Further experiments with alcohols revealed that butanol, hexanol, octanol and decanol also significantly inhibited 45Ca(2+) effluxes. However, undecanol and dodecanol did not cause any significant change on 45Ca(2+) fluxes, indicating that the effects of alcohols on 45Ca(2+) effluxes exhibit a cut-off phenomenon. In radioligand binding studies, it was found that at the concentrations used in flux studies, alcohols did not alter the characteristics of the specific binding of [3H]PN 200-110 to T-tubule membranes. Results indicate that ethanol directly inhibits the function of voltage-dependent Ca(2+) channels without modulating the specific binding of Ca(2+) channel ligands of the dihydropyridine class, and that this inhibition is independent of intracellular Ca(2+) levels.

Endogenous cannabinoid anandamide directly inhibits voltage-dependent Ca(2+) fluxes in rabbit T-tubule membranes

The effect of the endogenous cannabinoid, anandamide on Ca(2+) flux responses mediated by voltage-dependent Ca(2+) channels was studied in transverse tubule membrane vesicles from rabbit skeletal muscle. Vesicles were loaded with 45Ca(2+) and membrane potentials were generated by establishing K(+) gradients across the vesicle using the ionophore, valinomycin. Anandamide, in the range of 1-100 microM, inhibited depolarization-induced efflux responses. Anandamide also functionally modulated the effects of nifedipine (1-10 microM) and Bay K 8644 (1 microM) on Ca(2+) flux responses. Pretreatment with the specific cannabinoid receptor antagonist, SR141716A (1 microM), pertussis toxin (5 microg/ml), the amidohydrolase inhibitor, phenylmethylsulfonyl fluoride (0.2 mM) or the cyclooxygenase inhibitor, indomethacin (5 microM) did not alter the inhibition of efflux responses by anandamide. Arachidonic acid (10-100 microM) also effectively inhibited 45Ca(2+) efflux from membrane vesicles. In radioligand binding studies, it was found that both anandamide and arachidonic acid inhibited the specific binding of [3H]PN 200-110 to transverse tubule membranes with IC(50) values of 4.4+/-0. 7 and 13.4+/-3.5 microM, respectively. These results indicate that anandamide, independent of cannabinoid receptor activation, directly inhibits the function of voltage-dependent calcium channels and modulates the specific binding of calcium channel ligands of the dihydropyridine class.

A sustained inward current activated at the diastolic potential range in rabbit sino-atrial node cells

1. After blocking both the hyperpolarization-activated current and the membrane K+ conductance, depolarizations from -80 mV to between -70 and -50 mV induced a sustained current in sino-atrial node cells. We have tentatively designated this current Ist. 2. Ist was blocked by both organic and inorganic Ca2+ channel blockers, but was insensitive to tetrodotoxin (30 microM). Isoprenaline increased Ist. 3. The peak of Ist (at about -50 mV) was increased to 149 +/- 13% (n = 8, P < 0.01) by lowering the external Ca2+ concentration ([Ca2+]o) from 1.8 to 0.1 mM, in contrast to the marked depression of the L-type Ca2+ current. In 0.1 mM [Ca2+]o solution, the nicardipine-sensitive current-voltage relation showed the peak amplitude at about -50 mV and a reversal potential of +37.4 +/- 4.4 mV (n = 5). The isoprenaline-sensitive component also showed a reversal potential of about +30 mV. 4. Reducing [Na+]o from 140 to 70 mM in 0.1 mM [Ca2+]o decreased Ist to 53 +/- 5% (n = 7, P < 0.01). Increasing [Ca2+]o or [Mg2+]o decreased the amplitude of Ist. The half-maximum concentration of Mg2+ inhibition was 2.2 mM. 5. At 1.8 mM [Ca2+]o, Ist was inactivated by clamping for 5s at -10 mV, and gradually recovered after repolarization to -80 mV with a half-time of 1.36 +/- 0.4 s (n = 6). 6. The transitional sino-atrial node cell had minimal amplitude of Ist. 7. These characteristics of Ist are qualitatively comparable to those of the monovalent cation conductance of the L-type Ca2+ channel induced by depleting external divalent cations to the micromolar range. We conclude that Ist is generated by a novel subtype of L-type Ca2+ channel.

Stimulation of the slow calcium current in bullfrog skeletal muscle fibers by cAMP and cGMP

The effects of adenosine 3',5'-cyclic monophosphate (cAMP) and guanosine 3',5'-cyclic monophosphate (cGMP) on slow calcium currents (ICa) were investigated using the Vaseline-gap voltage-clamp technique in bullfrog skeletal muscle cut fibers. Both cAMP and cGMP induced a pronounced increase in the amplitude of ICa when applied to the cut ends of fibers. Both cyclic nucleotides also decreased time to peak current at all membrane potentials. The current-voltage relationship was shifted toward more negative potentials by cAMP as well as cGMP. The potentiating effects of cAMP and cGMP on ICa were additive. 8-Bromo analogues of both nucleotides had similar effects on ICa. The beta-adrenergic agonist isoproterenol, applied extracellularly, also produced an increase in the amplitude of ICa and produced a leftward shift in the current-voltage relationship. These results suggest that both cAMP and cGMP modulate calcium slow channels in bullfrog skeletal muscle fibers, causing stimulation of the ICa. The effect of cyclic nucleotides on ICa in bullfrog skeletal muscle contrasts with that in mammalian cardiac muscle, in which the same nucleotides produce opposite effects on the slow ICa, i.e., in cardiac muscle cAMP stimulates, and cGMP inhibits, the slow ICa.

Modulation of calcium current gating in frog skeletal muscle by conditioning depolarization

1. Ca2+ inward currents were measured by voltage clamping cut skeletal muscle fibres of the frog (Rana esculenta) in a double-Vaseline-gap system. 2. In order to study the basis of the previously described fast gating mode induced in the Ca2+ inward current by a conditioning depolarization we quantitatively analysed the response to differing features of the conditioning prepulse. 3. The faster activation seen during the second of two depolarizations was confined to the component of the inward current which could be blocked by 5 to 10 microM nifedipine. 4. By applying depolarizing conditioning pulses of gradually increasing length the time course of the transition to the fast gating mode could be determined. 5. Both the transition to the fast gating mode (point 4) caused by a depolarization and the slow inward current activated during the same depolarization showed similar voltage-dependent kinetics. 6. The kinetic change of the test current appeared to be equal when the same fractional activation was achieved at the end of the conditioning pulse independent of its duration or amplitude. 7. Flash photolysis of nifedipine in the interval between conditioning and test pulse showed that the predepolarization causes a rate-enhancing effect even though the slow channels were blocked by nifedipine during the conditioning pulse. 8. We conclude that the transition of the calcium channel from its slow to its fast gating mode is determined by the slow voltage-dependent reaction which limits the rate of channel opening under control conditions. This reaction is apparently not prevented by the binding of nifedipine and the block of current flow through the channel.

Inactivation of the slow calcium current in twitch skeletal muscle fibres of the frog

1. We investigated inactivation of the slow calcium current (ICa) at very positive potentials (over 30-40 mV) and recovery from inactivation in cut twitch skeletal muscle fibres of the frog, using the double-vaseline-gap technique. External solutions were buffered against changes in [Ca2+] (Ca(2+)-buffered) with malate. Internal solutions were Ca(2+)-buffered with high concentrations of either EGTA (60 mM) or BAPTA (30 mM). 2. ICa decayed to a steady-state level somewhat less than zero. Inactivation was most rapid at a potential 10 mV more negative than that which elicited the maximal ICa. 3. Involvement of current-dependent processes (i.e. tubular Ca2+ depletion and Ca2+ entry-dependent inactivation) in determining the decay of ICa was excluded, since inactivation was not affected by replacing Ca2+ with Ba2+ or when the size of ICa was reduced by decreasing the [Ca2+]o. Partial block of Ca2+ channels with nifedipine slowed inactivation. This was, however, independent of the size of ICa. Furthermore, neither the peak of ICa nor its time constant of decay nor the time course of ICa recovery from inactivation were affected by changing the [Ca2+]i from pCa 10 to 6. 4. ICa was potentiated during a post-pulse preceded by a pre-pulse at potentials ranging from -60 to -30 mV, whereas a U-shaped inactivation curve was observed at pre-pulse potentials more positive than -30 mV. This curve was asymmetric, since the ascending branch stabilized at a level less than unity. The U-shaped form of the curve depended on post-pulse voltage: it became more pronounced when the post-pulse depolarization increased. Moreover, the activation and inactivation kinetics of ICa during the post-pulse differed from control values. Similar results were found when Ca2+ was replaced with Ba2+. 5. The ICa recovery from inactivation was voltage dependent from -50 to -80 mV; it was voltage independent at more negative potentials, proving that recovery includes a voltage-independent step. 6. The asymmetric U-shaped inactivation curve can be reproduced by a four-state cyclic model without assuming a Ca(2+)-dependent step. Taking into account that recovery from inactivation includes a voltage-independent step which becomes rate limiting at extreme negative potentials, and that during the post-pulse the activation kinetics is faster, we propose a model which has six states, two closed, one open and three inactivated.

Voltage-dependent inactivation of T-tubular skeletal calcium channels in planar lipid bilayers

Single-channel properties of dihydropyridine (DHP)-sensitive calcium channels isolated from transverse tubular (T-tube) membrane of skeletal muscle were explored. Single-channel activity was recorded in planar lipid bilayers after fusion of highly purified rabbit T-tube microsomes. Two populations of DHP-sensitive calcium channels were identified. One type of channel (noninactivating) was active (2 microM +/- Bay K 8644) at steady-state membrane potentials and has been studied in other laboratories. The second type of channel (inactivating) was transiently activated during voltage pulses and had a very low open probability (Po) at steady-state membrane potentials. Inactivating channel activity was observed in 47.3% of the experiments (n = 84 bilayers). The nonstationary kinetics of this channel was determined using a standard voltage pulse (HP = -50 mV, pulse to 0 mV). The time constant (tau) of channel activation was 23 ms. During the mV). The time constant (tau) of channel activation was 23 ms. During the pulse, channel activity decayed (inactivated) with a tau of 3.7 s. Noninactivating single-channel activity was well described by a model with two open and two closed states. Inactivating channel activity was described by the same model with the addition of an inactivated state as proposed for cardiac muscle. The single-channel properties were compared with the kinetics of DHP-sensitive inward calcium currents (ICa) measured at the cellular level. Our results support the hypothesis that voltage-dependent inactivation of single DHP-sensitive channels contributes to the decay of ICa.

Fast gating kinetics of the slow Ca2+ current in cut skeletal muscle fibres of the frog

1. Calcium currents and intramembrane charge movements were measured in cut twitch muscle fibres of the frog and the time course of activation of the current was studied using various conditioning pulse protocols. 2. When a conditioning activation was produced by a depolarizing pulse which ended before inactivation occurred, a subsequent depolarization led to a faster onset of activation, indicating that the system had not completely returned to the initial state during the interval between the two pulses. 3. The interval between conditioning and test pulse was varied at different subthreshold potentials to study the time course of restoring the steady-state conditions. Complete restoration required a waiting period of about 1 min at the holding potential of -80 mV due to a very slow process but partial recovery was reached within 100 ms. This initial recovery process was strongly voltage dependent and became considerably slower when the interval potential approached the threshold for current activation. 4. Stepping to a roughly 10 mV subthreshold potential without applying a conditioning activation caused no change in the time course of the current produced by a subsequent test depolarization. Depolarizing just to the current threshold caused a slowly progressing acceleration of test current activation. 5. The peak current-voltage relation in the fast gating regime caused by a conditioning activation coincided with the current-voltage relation measured under steady-state conditions, indicating not that a new channel population had become activated but that the same channels showed a different gating behaviour. 6. Intramembrane charge movements measured in 2 mM-Cd2+ and tested at potentials between -40 and +40 mV showed negligible changes when preceded by a strong depolarization. 7. We discuss several possible models which can explain the fact that the current is speeded up by a conditioning activation while the charge movements remain unchanged. It is possible that the fast voltage-dependent transition which becomes visible after conditioning pulses reflects a rapid conformational change of the Ca2+ channel molecule which also occurs during its normal gating mode but remains undetectable in terms of conductance. In view of the hypothesis that the Ca2+ channel molecule forms a voltage sensor for excitation-contraction coupling this fast transition could be coupled to the control of Ca2+ release from the sarcoplasmic reticulum.

Calcium currents in embryonic and neonatal mammalian skeletal muscle

The whole-cell patch-clamp technique was used to study the properties of inward ionic currents found in primary cultures of rat and mouse skeletal myotubes and in freshly dissociated fibers of the flexor digitorum brevis muscle of rats. In each of these cell types, test depolarizations from the holding potential (-80 or -90 mV) elicited three distinct inward currents: a sodium current (INa) and two calcium currents. INa was the dominant inward current: under physiological conditions, the maximum inward INa was estimated to be at least 30-fold larger than either of the calcium currents. The two calcium currents have been termed Ifast and Islow, corresponding to their relative rates of activation. Ifast was activated by test depolarizations to around -40 mV and above, peaked in 10-20 ms, and decayed to baseline in 50-100 ms. Islow was activated by depolarizations to approximately 0 mV and above, peaked in 50-150 ms, and decayed little during a 200-ms test pulse. Ifast was inactivated by brief, moderate depolarizations; for a 1-s change in holding potential, half-inactivation occurred at -55 to -45 mV and complete inactivation occurred at -40 to -30 mV. Similar changes in holding potential had no effect on Islow. Islow was, however, inactivated by brief, strong depolarizations (e.g., 0 mV for 2 s) or maintained, moderate depolarizations (e.g., -40 mV for 60 s). Substitution of barium for calcium had little effect on the magnitude or time course of either Ifast or Islow. The same substitution shifted the activation curve for Islow approximately 10 mV in the hyperpolarizing direction without affecting the activation of Ifast. At low concentrations (50 microM), cadmium preferentially blocked Islow compared with Ifast, while at high concentrations (1 mM), it blocked both Ifast and Islow completely. The dihydropyridine calcium channel antagonist (+)-PN 200-110 (1 microM) caused a nearly complete block of Islow without affecting Ifast. At a holding potential of -80 mV, the half-maximal blocking concentration (K0.5) for the block of Islow by (+)-PN 200-110 was 182 nM. At depolarized holding potentials that inactivated Islow by 35-65%, K0.5 decreased to 5.5 nM.

Effect of Ca2+ channel blockers on K+ contractures in twitch fibres of the frog (Rana pipiens)

1. The effects of Ca2+ channel blockers (nifedipine, nitrendipine and diltiazem) were tested on K+ contractures in single muscle fibres of the frog, Rana pipiens. 2. Nifedipine (1 microM) reduced the area under K+ contractures to 24 +/- 9% (4) (100 mM-K+) and 34 +/- 24% (4) (190 mM-K+). Nitrendipine (0.1 microM) reduced the area to 30 +/- 10% (4) (120 mM-K+). The blockade of the contractures was reversible. 3. Diltiazem (1 microM) shortened the first 190 mM-K+ contracture without affecting the peak amplitude. The first contractures, performed at 15-20 min after the removal of diltiazem, were greatly reduced to 29 +/- 14% (4). This effect was reversed after three to five contractures in the absence of the drug. Similar results were obtained with 60 and 100 mM-K+. 4. The resting potential in control saline and after a brief exposure to 120 mM-K+ was not affected by the dihydropyridines and diltiazem. 5. Slow and fast Ca2+ currents were not modified by 1 microM-diltiazem at any stimulation rate or with pre-pulse depolarizations. Diltiazem (50 microM) did not affect the fast Ca2+ current and reduced the slow one to 48 +/- 10% (4). 7. The reduction of K+ contractures by Ca2+ channel blocking agents was not related to a blockade of Ca2+ currents. This can be tentatively explained by interactions of these compounds on membranes sites which regulate the coupling between membrane depolarization and contraction.